Electronic Devices and Systems

ABSTRACT

A device having a switched impedance that can be switched between a first state and a second state wherein, in the first state, the device acts as a voltage multiplier and, in the second state, the device acts as a rectifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of, and incorporates byreference the entire disclosure of, U.S. patent application Ser. No.10/525,408, which has been accorded a filing date of Apr. 17, 2006. U.S.patent application Ser. No. 10/525,408 is a national-stage filing ofPCT/AU2003/001072, filed Aug. 23, 2003.

FIELD OF INVENTION

In arrangements the invention has been developed primarily as a radiofrequency identification (“RFID”) tag for a parcel, document or postalhandling system and will be described hereinafter with reference tothese applications. However, the invention is not limited to thoseparticular fields of use and is also suitable to inventory management,stock control systems, and other applications.

The present invention provides novel and inventive electronic devices.

BACKGROUND ART

Passive RFID tags are known, and generally include a resonant tunedantenna coil electrically connected to an integrated circuit (IC).Examples of such RFID tags include: U.S. Pat. No. 5,517,194 (Carroll etal); U.S. Pat. No. 4,546,241 (Walton); U.S. Pat. No. 5,550,536 (Flaxel);and U.S. Pat. No. 5,153,583 (Murdoch).

Systems that employ RFID typically include an interrogator thatgenerates a magnetic field at the resonant frequency of the tunedantenna coil. When the coil is located within the magnetic field, thetwo couple and a voltage is generated in the coil. The voltage in thecoil is magnified by the coil's Q factor and provides electrical powerto the IC. With this power, the IC is thereby able to generate a codedidentification signal that is ultimately transmitted to theinterrogator.

Limitations arise because the resonant current that flows in the tunedantenna coil also generates a magnetic field in the region of the coil.That is, if there is an object—such as a second tag with a secondcoil—disposed near the first coil, the voltage generated by the firstcoil (and the second coil as well) will be reduced by the partialcancellation—or even complete cancellation—of these respective fields.In turn, this consequential reduction in power will not allow the firsttag (and likely the second tag as well) to reliably provide anidentification signal to the interrogator.

In this light, many fields that employ such tags—such as baggagehandling services, letter carrying services, inventory managementsystems, etc.—cannot be processed in “dense” configurations. In otherwords, such articles must be sufficiently spread apart for the tags—andsystems incorporating such tags—to operate reliably. Such “density”limitations thus tend to result in speed and efficiency restrictions.

In order to address these problems several inventive arrangements havebeen developed. As mentioned above and whilst these arrangements havebeen developed with particular regard to radio frequency identificationsystems that clearly also have advantageous application in a number ofother applications.

The discussion of the prior art within this specification is to assistthe addressee understand the invention and is not an admission of theextent of the common general knowledge in the field of the invention.

SUMMARY OF INVENTION

According to a first aspect of the invention there is provided a devicehaving a switched impedance that can be switched between a first stateand a second state wherein, in the first state, the device acts as avoltage multiplier and, in the second state, the device acts as arectifier.

Preferably in the first state the device acts as a voltage doubler andin the second state the device acts as a full wave bridge rectifier.Furthermore the switched impedance preferably comprises a switch inseries with a capacitor.

In preferred embodiment, the full wave bridge rectifier comprises aWheatstone bridge diode arrangement having a first and a second inputand a first and a second output, with the switched impedance beingconnected between a first input terminal and a first output terminal ofthe diode arrangement. As such the switched impedance may be connectedbetween a first input terminal and a first output terminal of therectifier.

In the ideal the voltage doubler has a voltage gain of two, andtransforms the load impedance by a factor of 8. In contrast, the fullwave rectifier, in the ideal, has a voltage gain of one, and transformsthe load impedance by a factor of 2. Thus, in arrangements, whenembodied in the form of a RFID device with an antenna coil, the voltagedoubler circuit is arranged to draw a significantly larger current fromthe antenna coil, and acts as the normal current state rectifier whilst,in contrast, when the full wave rectifier is switched “on” during a lowcurrent state significantly less current is drawn.

Other aspects and preferred aspects are disclosed in the specificationand/or defined in the appended claims, forming a part of the descriptionof the invention.

For example according to a second aspect of the invention there isprovided a method of selectively controlling current, the methodcomprising providing a device operable in one of a first state or asecond state, coupling an impedance to the device to enable theswitching of the device between a first state and a second statewherein, in the first state, the device acts as a voltage multiplierand, in the second state, the device acts as a rectifier. The method mayinclude using a receiving means to receive a signal, and selectivelycontrolling the amount of current in the receiving means by switchingbetween the first state and the second state. Further preferred featuresof the invention will become apparent.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a symbolic circuit diagram of a preferred embodiment of theinvention;

FIG. 2 is a symbolic circuit diagram of the embodiment shown in FIG. 1;

FIG. 3 is a symbolic circuit diagram of a voltage doubler circuitassociated with the embodiment of the invention shown in FIGS. 1 and 2;

FIG. 4 is a schematic representation of a device according to anotherpreferred embodiment of a related invention;

FIG. 5 is a plan view of the embodiment of FIG. 4;

FIG. 6 is a symbolic circuit diagram in one form illustrating a typicalprior art tag;

FIG. 7 is a symbolic circuit diagram of an RFID device according to theembodiment shown in FIG. 4;

FIG. 8 is a circuit model for the device of FIG. 7;

FIG. 9 is a symbolic circuit diagram of another embodiment of theinvention that includes a voltage multiplier;

FIG. 10 is a symbolic circuit diagram of a further alternativeembodiment of the invention that includes a circuit for changing thecurrent collection efficiency of the antenna;

FIG. 11 is a symbolic circuit diagram of a further embodiment of theinvention where the circuit for changing the current collectionefficiency is on the DC side;

FIG. 12 is a symbolic circuit diagram of another embodiment of theinvention that includes a circuit for changing the operating voltage;

FIG. 13 is a symbolic circuit diagram of a further embodiment of theinvention that includes a series voltage regulator circuit;

FIG. 14 is an alternative symbolic embodiment of that of FIG. 7, wherethe antenna coil is substituted with a generic interrogationsignal-receiving device;

FIG. 15 is an alternative symbolic embodiment of that of FIG. 7; wherethe antenna coil is substituted with a dipole antenna;

FIG. 16 is an alternative symbolic embodiment of that of FIG. 7; wherethe antenna coil is substituted with a capacitive antenna;

FIG. 17 is a circuit model for the prior art circuit of FIG. 6;

FIG. 18 is a perspective view of a plurality of stacked envelopes, eachof which contains a device according to FIG. 7;

FIG. 19 is a perspective cut-away view of a parcel according to anotherembodiment; and

FIG. 20 is a schematic representation of a system according to a furtherembodiment.

DETAILED DESCRIPTION

FIGS. 1 to 3

It is to be noted at the outset that FIGS. 1 to 3 are “symbolic” ormodels of a preferred embodiment of the invention.

Referring to FIG. 1 there is provided a device 100 having a switchedimpedance 101 that can be switched between a first state and a secondstate wherein, in the first state, the device 100 acts as a voltagemultiplier and, in the second state, the device 100 acts as a rectifier.In this particular embodiment the voltage multiplier, in the first stateof operation, is a voltage doubler and the rectifier, in the secondstate of operation, is a full wave bridge rectifier.

In the device 100 there is provided a switched impedance 101 comprisinga switch T1 in series with a capacitor C3. The switch T1 is able to bemoved between on and off states so as to switch the device 100 betweenthe first and second states. In the first state the switch T1 is closedwhile in the second state the switch T1 is open.

With T1 open no current can flow through capacitor C3 causing the device100 to act as a full wave bridge rectifier. The full wave bridgerectifier includes four diodes D1 to D4 arranged in a Wheatstone bridgeformation. In the formation as shown in FIG. 1 the tails of D1 and D4extend from a first output terminal 102 of the bridge to respectivefirst and second input terminals 104, 106, from which the tails of D1and D4 respectively extend to a second output terminal 108 of thebridge. This formation is known as a Wheatstone bridge.

With T1 closed the operation of the device 100 is best understood byrecasting FIG. 1 into the form shown in FIG. 2. Taking the effort torecast FIG. 1 and with subsequent circuit analysis, with switch T1closed, there is provided a voltage doubler in which the diodes D3 andD4, effectively, can be thought of being reversed biased so as toconduct no current. Removing D3 and D4 from FIG. 2 consequently resultsin a voltage doubler arrangement. Moreover, as is know, a voltagedoubler of this type, in the ideal, transforms the load impedance by afactor of 8. With switch T1 open, the full wave bridge rectifier willtransform the load impedance by a factor only 2. The advantageous natureof this construction for RFID devices is discussed below.

The importance of another feature of the device 100 is that with theswitched impedance 101 being connected between the input terminal 106and the output terminal 102 of the bridge, there is provided for theeffective reuse of diodes D1 and D2 in the second state of operation asa voltage doubler.

FIG. 4

It will now become apparent that the device 100 includes both a voltagedoubler circuit and a full wave rectifier circuit, and finds particularapplication in RFID devices.

In the arrangement schematically represented in FIG. 4 the presentinvention is embodied as a radio frequency identification device 200.The device 200 comprises a receiver portion 235; an integrated circuit237 with one or more functionalities; a connection 239 between the two;and a state selection means 241 that determines whether the device is ina first state or a second state; and a transmission means 245—preferablyin the form of an antenna 247. These components are reflectedsymbolically in the following Figures.

FIG. 5

A radio frequency identification (“RFID”) device or tag 1, issymbolically illustrated in FIG. 5. The tag 1 includes a multi-turn coil303 for receiving an interrogation signal. A transceiver, in the form ofan integrated circuit (IC) 304, is connected to the coil 303 and isresponsive to the interrogation signal. In other embodiments, otherdevices are used as the transceiver; such devices will be readilyapparent to those skilled in the art. In this embodiment, coil 303 andthe circuit 304 are mounted on a common generally rectangular substrate302. The IC includes a memory 306. The device 100 is included in the tag1.

FIG. 6

FIG. 6 includes a tuning capacitor C1. This Figure is discussed below inrelation to a specific example.

FIG. 7

As schematically illustrated in FIGS. 7 the circuit 304 toggles betweena first state and a second state, wherein the current drawn from thecoil 303 by the circuit 304—in the presence of the interrogationsignal—during a first state is greater than the current drawn during asecond state. A noted above device 100 is employed within circuit.

FIG. 8

FIG. 8 illustrates a circuit model for circuit 304. Particularly:

-   (a) the voltage V1 is induced in the antenna coil L1 by the    interrogation field.-   (b) Impedance Z1 represents the series impedance of the antenna coil    and any other series—connected impedance.-   (c) R4 symbolically represents the equivalent AC resistance of    circuit 304.-   (d) Current I2 flows from the antenna coil into R4.-   (e) Voltage V4 across R4 symbolically represents the voltage of the    antenna terminals of L1 and circuit 304, which is rectified and    stored on a DC storage capacitor C2 as shown in FIG. 3.

Accordingly, V4 equals V1 minus the volt drop in L1 and Z1 due to thecurrent I2 flowing through L1 and Z1. That is:V4=V1−I2.(Z1+jwL1)

Where jw is the complex frequency in radians per second. This equationcan be rearranged into the following two forms.I2=(V1−V4)/(Z1+jwL1)andI2=V1/(R4+Z1+jwL1)Adjusting I2

In light of the above, assuming that the voltage V1 and the inductanceL1 is fixed, then current I2 is adjusted by varying either V4, R4 or Z1.For instance:

-   1. I2 is varied by changing V4. That is, by increasing the output    voltage more voltage appears at the coil terminals and less current    is drawn from the antenna coil.-   2. I2 is varied by changing R4. That is, by increasing the AC    resistance of the circuit 4 less current is drawn from the antenna    coil. And,-   3. I2 is varied by changing Z1. That is, by inserting an extra    impedance in series with Z1, a larger voltage is dropped in the    antenna coil impedance and less current is drawn from the antenna    coil.

Embodiments incorporating such techniques have in fact already beendescribed in the context of device 100 represented in FIGS. 1 to 3.

Resonant Tuning

As described with T1 closed in device 100 there is provided a voltagedoubler of the form illustrated in FIG. 3. With reference to theembodiment shown in FIG. 7 this state of operation can be viewed ascomprising a voltage multiplier as shown in FIG. 9. This isadvantageous, since in the absence of resonant tuning, the coil voltageis relatively low because it is not magnified by Q. To compensate,circuit 8 increases the voltage supplied to circuit 7 and allows thecircuit to operate with a lower coil voltage; the lower coil voltagealso requiring a lower interrogation field.

In other related devices use is made of other types of voltagemultipliers, such as triplers or quadruplers. Since the impedance levelof the coil used in many preferred embodiments is low—in the order of200 ohms—it is, therefore, ideally suited to a connection with a voltagemultiplier.

Load Impedance

As discussed above the voltage doubler has a voltage gain of two, andtransforms the load impedance of the chip by a factor of 8. In contrast,the full wave rectifier has a voltage gain of one, and transforms theload impedance by a factor of 2. Thus, since the voltage doubler circuitdraws a significantly larger current from the antenna coil, it acts asthe normal current state rectifier. In contrast, the full wave rectifieris switched “on” during the low current state.

In the case of the full wave rectifier, for an AC input voltage of Vacpeak (2Vac peak to peak) the DC output voltage Vdc equals Vac. For a DCload resistance Rdc the output power Pout equals Vdcˆ2/Rdc and the inputpower Pin equals Vacˆ2/2.Rac where Rac is the input impedance. Again,from energy conservation the input power Pin must equal the output powerrequiring that Vdcˆ2)/Rdc=Vacˆ2/2.Rac. Substituting Vdc=Vac gives(Vacˆ2)/Rdc=Vacˆ2/2.Rac and rearranging gives 2.Rac equalling Rdc. Thusthe input AC resistance is ½ of the DC load resistance.

The switch T1 is provided in the form of a MOSFET transistor and aswould now be apparent, is used to select either the normal current stateor the low current state. (T1's drive is provided by the transceiver.)When transistor T1 is closed and opened, the circuit respectively actsas a voltage doubler and a full wave rectifier.

In the present embodiment circuit 304, has a current cycle during whichthe circuit randomly selects either the first or the second state forthe duration of the cycle. The random selection of state during thecycle by each individual tag reduces the risk of two adjacent radiofrequency identification tags simultaneously operating in the firststate.

Moreover, in this embodiment, the selection of the second state bycircuit 4 is about 16 times more probable then the selection of thefirst state. That is, the probability of the circuit 4 drawing a highcurrent—and thereby jeopardizing the performance of an adjacent tag, anditself, by their mutual coupling is 1/16. Accordingly, the tags mayoperate at a much smaller spatial separation than could be achieved byprior art tags.

The state selection means is implemented with digital circuits. Thesecircuits are designed to select the current state according to thechosen algorithm or method. There are several methods which can be usedto implement the state selection circuits. Logic gates can be used tocreate a dedicated logic circuit for determining the state selection. Astate engine consisting of logic arrays can be designed to implement thestate selection function. A microcontroller or processor can executesoftware instructions that code for the chosen algorithm or method. Thepreferred embodiment is a logic array controlled by a microcontroller.The microcontroller software executed the slower parts of the chosenalgorithm or method while the logic array performs the faster parts ofthe chosen algorithm or method.

A. Embodiments with an Extra Impedance

In FIG. 10, circuit 11 includes a sub-circuit 12 that provides an extraimpedance Z2 in series with the antenna coil L1 when circuit 11 is inthe low current state. Z2 can be a resistance, capacitance, inductanceor a combination of any, or all, of these. The extra impedance causes adrop in voltage across itself and reduces I2. This is advantageous forreducing the current drawn from the antenna during the low currentstate.

In other embodiments, such as that shown in FIG. 11, circuit I2 isplaced on the DC side of the rectifier and a resistor R3 is used toreduce I2.

B. Embodiments with a Shunt Regulator

The embodiment shown in FIG. 12 includes a circuit 15 that utilises ashunt regulator 16 for controlling the operating voltage provided to theintegrated circuit. A detailed explanation of the operation of the shuntcircuit is given in U.S. Pat. No. 5,045,770.

In essence, the IC's operating voltage is changed such that the lowcurrent state's operating voltage, VA+VB, is higher than the normalcurrent state's operating voltage, VB. When the IC's is at the higheroperating voltage, the transceiver portion of the device operates at alower current—therefore, less current is drawn from the antenna.

The low current state operating voltage is set as high as is possiblegiven the limitations of the IC technology. In this embodiment, forexample, VA+VB=4.2 volts and VB=2.1 volts.

C. Embodiments with a Series Regulator

The embodiment of FIG. 13 includes a circuit that utilises a seriesregulator for controlling the operating voltage. The input voltage tothe regulator increases when the circuit toggles into the low currentstate.

Dimensions

Returning to FIG. 5 the substrate 302 is about 80 mm by 50 mm, andincludes a plurality of layers that are laminated together toencapsulate the coil 303 and the circuit 304. In this embodiment, thethickness of the tag 1 is about 0.3 mm. In other embodiments, thedimensions of tag 1 are bigger or smaller. That is, it is generallypreferable for the tag to be sized such that it may be unobtrusivelyincorporated into packaging and other articles.

Devices Used to Transmit the Identification Signal

In the preferred embodiment, the coil 303 transmits an identificationsignal generated by the transceiver. In other embodiments, a secondseparate antenna coil is used to transmit the identification signal.

Devices Used to Receive the Interrogation Signal

While in this embodiment, the antenna is the coil 303, other devices maybe employed to receive the interrogation signal. Examples of suchalternative devices are shown in FIGS. 14, 15 and 16. In FIG. 14, theinterrogation signal is received by a non-specific or generic receivingdevice 31. As shown in FIG. 15 includes a dipole antenna 32 is used forreceiving a radiated interrogation signal. In other embodiments (notshown), device 31 is a monopole. In still further embodiments, such asthat illustrated in FIG. 18, device 31 includes a capacitive antenna 33for receiving an electric, capacitive, or interrogation signal. Further,it will be understood by the skilled addressee from the teaching hereinthat the invention is applicable to still other receiving devices, andis not limited by the choice of antenna or the specific form ofinterrogation signal.

The Typical Operation of Prior Art Tags

Before further describing the embodiments of the invention, theoperation of a typical prior art tag will be examined. A typical tagincludes a circuit 6 illustrated schematically in FIG. 6. Particularly,the voltage V1 is induced in antenna coil by the interrogation field,and the antenna coil L1 is tuned by a tuning capacitor C1. Accordingly,L1 and C1 form a resonant tuned circuit, which magnifies the voltage V1by the loaded Q factor of the antenna coil. The AC voltage generatedacross the tuned circuit is rectified by a rectifier 6 a, and the DCoutput voltage is stored on a storage capacitor C2. The DC load of theIC is represented by R1.

FIG. 17 shows a circuit model for the RFID circuit 5 where correspondingfeatures are denoted by corresponding notations The antenna coil isrepresented by inductance L1 and the coil losses by series resistanceR5. The tuning capacitance and circuit stray capacitance are representedby C1, and the losses of the rectifier and IC circuit by R3. Theresonant currents circulating in the tuned circuit formed by L1 and C1are I1; and the output current into R3 is I2.

The capacitor Q factor (Qc=w.R3.C1) normally dominates the totalresonant Q factor. Typically, Qc has a value of between 10 and 40. Sincethe ratio of I1/I2=Q, the resonant current I1 is much larger than theoutput current I2.

In light of the above, when tags of this type are in close proximity themagnetic field generated by the resonant current couples—through mutualinductance—with proximate tags and, therefore V1 is diminished. In otherwords, once the tags are in close proximity—that is, within about 50 mmof each other—such “interference” compromises the reliable operation ofthe tags.

The Removal of the Resonant Capacitor

In a related aspect it has been appreciated by the inventors that fortags operating in close proximity to each other it is important thatthese resonant currents are eliminated. Given this, the inventors havefound that it is possible to eliminate these resonant currents bydisconnecting the resonant capacitor from the antenna coil. However,even with the resonant capacitor removed from prior art devices likethat shown in FIG. 7, the antenna current drawn by circuit 7 is stilltoo large to allow a plurality of tags to be closely stacked.Specifically, even without a resonant capacitor, if such tags are placedwithin a few millimetres of each other, the tags will not operatereliably.

Minimising the Current in the Second State

When the antenna coil current becomes very small or, as in some caseszero, the coil becomes transparent to the interrogation field. In thisstate the antenna coil has (a) no effect upon the interrogation fieldand (b) those tags in the low current state do not interfere with theoperation of those tags in the normal current state.

In low current state, tag 1 is not fully functional. That is, thecurrent drawn from the coil is reduced such that only necessary circuitfunctions are viable. In a preferred embodiment, the current is in orderof 30 μA. Ideally, the current is zero; or at least minimised as much aspossible.

In other embodiments, the minimising of current is realised by one ormore of a variety of methodologies, including:

-   1. Minimising the required functions to be performed by the    circuitry.-   2. Utilising low power circuitry. Low power circuitry, while widely    understood, are much more difficult to design than conventional    circuitry. Low power circuits require less current to operate and    consequently draw less current. Using low power circuits for those    circuits that must remain operational in the low current state    reduces the current drawn during the low current state.-   3. The use of onboard energy storage devices and in particular a    capacitive device. On board storage devices can provide the current    required to operate the circuits in the low current state. For    example, a capacitive device can charge up during the normal current    state and use the stored charge during the low current state so as    to minimise the current drawn from the antenna. Alternatively, a    battery can be used to supply the low current state current.

More generally, the impedance seen by the antenna coil should be aslarge as possible. This is particularly so in the low current state.That is, the quantum of the antenna current is proportional to thequantum of the resistive and/or the reactive load as seen by the coil.When the amount of the coil current is too high, coil-to-coil magneticinterference will cause the tags to stop operating reliably.

Operation

In the FIG. 7 embodiment—which does not include a resonatingcapacitor—voltage V1 is induced in the antenna coil L1 by theinterrogation field. Further, the antenna voltage is rectified andstored on a DC storage capacitor C2. The generated current is managed bysymbolic switch SW1.

A. The Symbolic Switch

The two states can be symbolically reflected by a switch SW1 andresistors R1 and R2. Importantly, these are employed to reflect the twostates and are not, in fact, part of the invention.

In other words, switch SW1 reflects the device's operation in the twodifferent “states”. In essence, this is further symbolically implementedby resistors R1 and R2—which are representative of the load provided bycircuit 4 in the low current state and the normal current staterespectively.

With the benefit of the teaching herein, it will be appreciated by thoseskilled in the art that there are many well known methods for disablingcircuits and reducing their current consumption—all of which areapplicable to achieve the functionality required. For example, there arevarious hardware and software methods for putting a microprocessor intoa “standby” or a “sleep” state. The present invention does howeverprovide a solution whereby the device includes a switched impedance thatcan be switched between a first state and a second state wherein, in thefirst state, the device acts as a voltage multiplier and, in the secondstate, the device acts as a rectifier.

B. Current Input by the Symbolic Switch

The change in the current drawn by circuit 304 in the low current andthe normal current state corresponds to a change in the antenna coil'scurrent. In the low current state that antenna current is tens ofmicroamperes and in the normal current state the antenna current ishundreds of microamperes. Specifically, typical values are 70 μA in thelow current state and 300 μA in the normal current state.

In FIG. 7, the low current state is symbolically represented by switchSW1 being open and the current Iq being drawn through R2. In the lowcurrent state, the quiescent current Iq is symbolically drawn. Thecurrent Iq is very small and is typically a few tens of microamperes. Inthis embodiment, Iq symbolically represents the current used to:maintain RAM data stored in CMOS memory, operate logic functions, andpower analogue circuitry.

Further, the normal current state is symbolically represented by SW1being closed and reflects activation of all of circuit 4'sfunctionality. In the normal current state, currents Ic and Iq aredrawn. The total current drawn by circuit 304 in the normal currentstate (Iq+Ic) is typically about 300 μA, although this does varyconsiderably between embodiments.

Systems Incorporating the Device

FIG. 18 illustrates an application of an embodiment of the invention asan inventory system for jewels. Previously, this process has beenachieved manually, and is therefore both time consuming and prone toerror.

In this embodiment, 100 small envelopes are horizontally stacked in acardboard box; each envelope storing a jewel and a report on thecharacteristics of the jewel. As is evident from FIG. 18, a plurality ofRFID tags 1 may be placed within a few millimetres of each other withoutimpacting on the devices' reliability.

Since each tag 1 is programmed with the contained jewel'scharacteristics, its uniquely coded identification signal will providethe interrogator with data this is indicative not only of the identityof each tag in the box, but also of the jewel contained within eachenvelope. Accordingly, the whole box of jewels is accounted for in oneautomatic process. There is no need to take the envelopes out of the boxand separate them to “safe” distances from each other.

In this way, security is more easily maintained as well. For instance,the interrogator may be placed at a passage (through which the box isplaced) between a safety deposit storage area and a customer servicearea. Preferably, the personnel progressing the box also carries a tagso that their identity may be determined.

The Determination of “State”

As mentioned earlier, to maximise the reliability of the operation ofclosely stacked or spaced tags, such as those used in FIG. 18, the tagsoperate in either of two current states. At any one time, a smallproportion of the tags are in a normal current state where the tags areresponsive to the interrogator, and the remainder of the tags are in alow current state where they are not fully functional. Accordingly, inthe FIG. 18 embodiment, where the tags must operate within a fewmillimetres of each other, the probability of an individual tag being inthe normal state is 1/16.

Generally speaking, the longer the tags are disposed within theinterrogation field, the lower the normal state probability may be. Inother embodiments having only a few tags, the probability of the tagsbeing in the normal state can also be decreased. In such instances, thespacing between tags can thereby be further decreased as well.

The selection of state is made using a predetermined algorithm. Anexample of a preferred algorithm is a random or a pseudo-random numberalgorithm.

A. Autonomous Selection

In a preferred embodiment, the tags randomly select their current stateautonomously. That is, the tags randomly choose a current state; receivecommands and/or data, and/or transmit replies; and then randomly choosea new current state.

B. Responsiveness to Interrogation Signals

In alternative embodiments, the interrogation signals are used to directtags to select a new current state, and the tags randomly choose theircurrent state. These interrogation signals, in some embodiments, takethe form of short breaks in the interrogation field. Example of suchbreaks include a single break and a coded break (where the codes aresequences of breaks directing the tags to perform to various currentstate selection).

In further alternative embodiments, other forms of modulation of theinterrogation field are used to direct tags in their selection ofcurrent state. Examples of such modulations include amplitude, phase,and frequency modulation.

C. Probabilities

The precise proportion of tags selecting the normal state is notcritical, except in so far that the coupling between tags is reducedsufficiently to allow reliable operation. The probabilities orproportion of operating tags should be selected to suit the number andspacing of tags and can be determined by experiment.

Moreover, the algorithm may be structured so that a tag will beguaranteed to have been in the normal current state at least once every“n” state selections, where “n” is the reciprocal of the probability ofselecting the normal state. A simple method of ensuring this is to forcethe selection of the normal current state if it has not been selectedafter a fixed number of selections. The value of this fixed number canbe selected to suit the number and spacing of tags.

D. Use of Unique Tag Number

Alternatively, each tag selects a current state dependent upon a fixednumber, such as a unique number. In such preferred embodiments, the taguses a portion of that number to choose a current state. Moreparticularly, in the FIG. 18 embodiment, each tag's unique numberincludes a 4-bit mask value. The 4-bit value represents the number ofinterrogator breaks, or commands, received before the tag enters thenormal current state. The field transmitted by the interrogator can bemodulated to transmit commands to the tags. Various methods ofmodulating the field such as pulse, amplitude, frequency and phase arewidely used and understood.

In further embodiments, the mask may be altered each time the tag exitsthe normal state. In this way, adjacent tags with similar numbers areprevented from moving to the normal current state at the same time.

Larger and smaller probabilities can be selected by using smaller andlonger masks. The mask can also be reduced or increased in length sothat probabilities of 1, ½, ¼, ⅛, 1/16, and 1/32 can be selected byemploying masks of 0, 1, 2, 3, 4 and 5 bits respectively.

Another application is illustrated in FIG. 19, where tag 1 is showndisposed between two cut-away layers 21 and 22 of a laminated envelope23. While tag 1 is shown in the Figure as protruding from between thelayers, that is for purposes of illustration only. It will beappreciated that, in use, tag 1 is completely enclosed by the layers.Importantly, since tag 1 is operable, even when in close proximity to anumber of like tags, it is possible to reliably interrogate the tags.

Further Applications

FIG. 20 depicts a system 50 according to a preferred embodiment of theinvention. As shown, an interrogator 43 integrates a plurality ofdevices 1.

For postal envelopes, the user is able to pre-program the tags 1 toinclude address and content information to facilitate the sorting of theenvelope. Moreover, in some embodiments, the tag is pre-programmed withan encrypted message for the intended recipient. For courier envelopes,the courier may pre-program the tag to include data about the intendedrecipient, the contents of the envelope, the priority of the requireddelivery, and other data.

Although the tag 1 is shown sandwiched between two layers of theenvelope of FIG. 19, in other embodiments it is attached by other means.For example, one embodiment makes use of a plastics pocket formed on theexterior layer of the envelope for selectively receiving the tag. Inanother embodiment, the tag is simply placed within the envelope withthe other contents. Further, attached to parcels, the invention isparticularly advantageous because loosely packed parcels will often liedirectly adjacent to one another—without any separation. Otheralternatives will also be apparent to the skilled addressee in light ofthe teaching herein.

In another embodiment of the invention, a tag is disposed within thepackaging for a saleable item. Following the placement of the item intothe packaging the tag is programmed to include data indicative of thequantity or quality of the contents. This allows ease of distributionand inventory control from the point of packaging to the ultimate pointof sale. This embodiment is particularly advantageous when applied topackaging for computer software. However, it is also applicable to otheritems such as compact disc's, toys, integrated circuits, books, and anyother goods that are packed closely together for storage ortransportation.

In more complex embodiments, a number of tags are associated with asingle article. In the case of an envelope for courier use, one of thetags contains data readable only by the courier organisation, whileanother tag includes data only readable by the sender and recipient ofthe envelope.

The Interrogator

The interrogator 43 is either a fixed installation device or, in otherembodiments, a handheld device. In any event, the interrogator providesan interrogation signal—preferably in the form of a RF field—that isdetected by, and selectively responded to, by each tag in its field.

Reusability and Reliability

The RFID tags of the preferred embodiments provide a re-usable resource,as the tags are re-programmable. Moreover, unlike bar codes, they willnot be so easily disabled through physically rough handling.

Other Benefits Associated with the Present System

Since prior art system, tags are used to identify items such as baggageand are designed to operate at ranges up to 1 metre, the application ofsuch technology is thereby limited to circumstances where tags are wellspaced apart. In sharp contrast, the preferred embodiments of theinvention are able to be stacked closely and continue to reliablyoperate.

A typical application is the identification of RFID tags attached tobundles of letters where the tag data is used to control the automaticsorting of each letter. However, the invention is not limited to thisparticular field of use. For example, various aspects of the inventionare applicable to systems used for identification or inventorymanagement of items such as shoe uppers, shoe soles, diamonds, andjewellery.

Moreover, in addition to allowing ease of inventory control, theinvention facilitates the automated sorting of those articles. This iswell illustrated in the context of the jewel handling system and also inthe context of mail handling systems—where each piece of mail includes atag.

Accordingly, the preferred embodiments may be applied advantageously tovarious uses such as item identification, stock control, and inventorymanagement. By having the ability to reliably operate in “close” ranges,such as when stacked, the application's tag and system allow theseprocesses to be done in bulk and automatically—without the need formanual intervention. Accordingly, the preferred embodiments of theinvention provide many significant advantages over prior art systems.

It is to be appreciated that the present invention may be provided aradio frequency identification (“RFID”) device, the device including: anantenna for receiving an interrogation signal; and a transceiverconnected to the antenna and being responsive to the interrogationsignal, whereby the transceiver selectively draws current from theantenna.

The transceiver may toggle between a first state and a second state,wherein the current drawn by the transceiver during the first state isgreater than the current drawn during the second state. The transceivermay select the second state more frequently than the first state. Morepreferably, the probability of selecting the second state is at leasttwice the probability of selecting the first state.

In a preferred embodiment, the transceiver has an operating cyclewherein, during that cycle, the transceiver is in either the first orthe second state. Preferably, the transceiver selects the first statewith a probability of less than ½. More preferably, the probability isless than ¼.

Even more preferably, the probability is less than or equal to 1/16.Accordingly, the first state is not necessarily selected in each cycle.In signal use, the interrogation signal is generated in a predeterminedarea by an interrogator. Preferably, the device is maintained within thesignal field for more than one cycle. More preferably, the device ismaintained within the field for at least the number of cycles equal tothe reciprocal of the probability of the first state being selected.

In a preferred form, the selection of the first state and the secondstate is based upon a predetermined algorithm. An example of a preferredalgorithm is a random or a pseudo-random number.

Preferably, the antenna and the transceiver are mounted to a commonsubstrate. More preferably, the antenna is a coil and the currentgenerated in the coil is in response to the interrogating signal.

Preferably, during the first state, the current drawn by the transceiveris to allow its operation. That is, the first state is a normal state,while the second state is a standby state. For example, in the normalstate the current supplies the relevant clock circuits, the signalprocessing circuit, and the like. In this state, the current also allowsthe transceiver to generate an identification signal.

More preferably, the transceiver relies upon the current to drive theantenna to transmit the identification signal. In other embodiments, thedevice includes a separate transmission antenna and the transceiverdrives that separate antenna to transmit the identification signal. Inboth cases, the current drawn from the antenna is the source of powerfor the generation and transmission of the identification signal.

The device is preferably passive in that it does not have an onboardpower source.

However, the invention is also applicable to active devices wherein thelife of the onboard power source is prolonged.

The present invention may be provided as a radio frequencyidentification (“RFID”) device, the device including: an antenna forreceiving an interrogation signal and being responsive to the signal forsupporting an antenna current; a coupling connected to the antenna fortoggling the antenna current between a first state and a second state,wherein the antenna current in the first state is greater than theantenna current in the second state; and a transceiver connected to thecoupling and drawing an operational current that is derived from theantenna current, whereby the transceiver is selectively responsive tothe interrogation signal to generate an identification signal.

Preferably, during the first state the transceiver is responsive to theinterrogation signal to generate the identification signal. Morepreferably, in the second state the device is responsive to theinterrogation signal only for the purpose of toggling the antennacurrent between the first and second states. That is, the first state isa normal current state, whereas the second state is a low current orstandby state.

Preferably also, the antenna is responsive to the transceiver fortransmitting the identification signal. In other embodiments, however,the device includes a separate antenna that is responsive to thetransceiver for transmitting the identification signal.

The present invention may be provided as a system for identifyingarticles that are collocated with an RFID tag of the first aspect, thesystem including: an interrogator for providing an interrogating field;a plurality of identification devices mounted to the respectivearticles, the devices including: respective antennas for beingcontemporaneously disposed within the field and being responsive to thatfield for providing antenna currents; respective transceivers that areconnected to the antennas for selectively toggling the currents betweenan operational state and a standby state such that not all the currentsare simultaneously in the operational state, whereby the transceiversare responsive to the currents for providing identification signals thatinclude identification data unique to the respective articles; and areceiver for processing the identification signals to extract theidentification data and thereby identify the respective articles.

Preferably, the current drawn by the transceiver during the operationalstate is greater than the current drawn during the standby state. Morepreferably, the transceiver selects the standby state more frequentlythan the operational state. Even more preferable, the probability ofselecting the second state is at least twice the probability ofselecting the first state.

In the preferred embodiments, the transceiver has an operating cyclewith a start and a finish wherein, during that cycle, the transceiver isin either the first or the second state.

Preferably also, the transceiver selects the first state with a smallprobability of less than ½.

More preferably, the probability is less than ¼. Even more preferably,the probability is less than or equal to 1/16.

In a preferred form, the selection of state is based upon apredetermined algorithm. An example of a preferred algorithm is a randomor a pseudo-random number used to determine the state selection of thetransceiver.

Preferably, the identification signals are transmitted while therespective transceivers are in the first state. More preferably, thetransceivers use the respective antennas to transmit the identificationsignals. In other embodiments, however, the devices include respectivesecond antennas that are used by the transceivers to transmit theidentification signals.

The present invention may be provided as a radio frequencyidentification(“RFID”) device including: an antenna that is responsiveto an interrogation signal for providing an antenna current; and atransceiver for selecting between a normal state and a standby statewherein, during the normal state, the transceiver is responsive to theinterrogation signal for generating an identification signal and, duringthe standby state, the transceiver is only responsive to theinterrogation signal for selecting between the normal and standbystates.

Preferably, in the absence of the interrogation signal the device isinactive. Conversely, in the presence of an interrogation signal, thedevice is either in the normal state or the standby state. Preferably,the normal state has a short duration and, therefore, the device ispredominantly in the standby state in the presence of an interrogatingsignal. Preferably, during the standby state, the device is onlyresponsive to the interrogation signal for the purpose of selectingbetween normal and standby states.

The present invention may provided as a voltage regulator for a radiofrequency identification (“RFID”) device; the device having: an antennafor receiving an interrogation signal and for transmitting anidentification signal and a transceiver for being responsive to theinterrogation signal to generate the identification signal. Theregulator including: a current coupling for providing a supply voltageto the transceiver, the current coupling, in the first state, drawing afirst current from the antenna and, in the second state, drawing asecond current from the antenna that is less than the first current.

The present invention may provided as an identification device forreceiving a first signal and transmitting a second signal, the deviceincluding: a receiving means for receiving the first signal andemploying the first signal to generate a voltage; wherein the receivingmeans generates a first current from the voltage; an integrated circuitthat selectively controls the amount of the first current in thereceiving means; a connection between the receiving means and theintegrated circuit; a transmission means for generating the secondsignal; a state selection means for selecting whether the device is in afirst state or a second state; wherein-relative to the second state-arelatively larger amount of the first current flows through thereceiving means when the device is in the first state; andwherein-relative to the first state-a relatively smaller amount of thefirst current flows through the receiving means when the device is inthe second state.

The present invention may provided as system for identifying articles,the system including: a signal generator for generating a first signal;a plurality of articles; a plurality of identification devices, eachindividual device being respectively associated with each individualarticle; wherein each device includes: a receiving means for receivingthe first signal and employing the first signal to generate a voltage;wherein the receiving means generates a first current from the voltage;an integrated circuit that selectively controls the amount of the firstcurrent in the receiving means; a connection between the receiving meansand the integrated circuit; a transmission means for generating thesecond signal; a state selection means for selecting whether the deviceis in a first state or a second state; wherein-relative to the secondstate-a relatively larger amount of the first current flows through thereceiving means when the device is in the first state; andwherein-relative to the first state-a relatively smaller amount of thefirst current flows through the receiving means when the device is inthe second state.

Although the invention has been described with reference to a number ofspecific examples, it will be appreciated by those skilled in the artthat the invention can be embodied in many other forms.

While this invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification(s). This application is intended to cover any variationsuses or adaptations of the invention following in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth.

As the present invention may be embodied in several forms withoutdeparting from the spirit of the essential characteristics of theinvention, it should be understood that the above described embodimentsare not to limit the present invention unless otherwise specified, butrather should be construed broadly within the spirit and scope of theinvention as defined in the appended claims. Various modifications andequivalent arrangements are intended to be included within the spiritand scope of the invention and appended claims. Therefore, the specificembodiments are to be understood to be illustrative of the many ways inwhich the principles of the present invention may be practiced. In thefollowing claims, means-plus-function clauses are intended to coverstructures as performing the defined function and not only structuralequivalents, but also equivalent structures. For example, although anail and a screw may not be structural equivalents in that a nailemploys a cylindrical surface to secure wooden parts together, whereas ascrew employs a helical surface to secure wooden parts together, in theenvironment of fastening wooden parts, a nail and a screw are equivalentstructures.

“Comprises/comprising” when used in this specification is taken tospecify the presence of stated features, integers, steps or componentsbut does not preclude the presence or addition of one or more otherfeatures, integers, steps, components or groups thereof.” Thus, unlessthe context clearly requires otherwise, throughout the description andthe claims, the words ‘comprise’, ‘comprising’, and the like are to beconstrued in an inclusive sense as opposed to an exclusive or exhaustivesense; that is to say, in the sense of “including, but not limited to”.

1. A device having a switched impedance that can be switched between afirst state and a second state wherein, in the first state, the deviceacts as a voltage multiplier and, in the second state, the device actsas a rectifier.
 2. A device as claimed in claim 1 wherein the deviceincludes receiving means for receiving a signal, the device beingconfigured for selectively controlling the amount of current in thereceiving means by switching between the first state and the secondstate.
 3. A device as claimed in claim 2 wherein the current drawnthrough the receiving means in the first state is greater than thecurrent drawn through the receiving means in the second state.
 4. Adevice as claimed in claim 2 or 3 wherein the receiving means isconnected to one or more voltage input terminals of the device.
 5. Adevice as claimed in claim 2 wherein the receiving means comprises acoil in an antenna circuit.
 6. A device as claimed in claim 2 whereinthe receiving means has a impedance of substantially 200 ohms.
 7. Adevice as claimed in claim 1 wherein a load is connected between one ormore output voltage terminals of the rectifier.
 8. A device as claimedin claim 7 wherein the load comprises an integrated circuit.
 9. A deviceas claimed in claim 1 wherein the first state is an operational stateand the second state is a standby state.
 10. A device as claimed inclaim 1 wherein the switched impedance comprises a switch in series witha capacitor.
 11. A device as claimed in claim 10 wherein the switch ofthe switched impedance comprises a MOSFET.
 12. A device as claimed inclaim 1 wherein the switched impedance is connected between a firstinput terminal and a first output terminal of the rectifier.
 13. Adevice as claimed in claim 1 wherein the device includes a seriesregulator for controlling an operating voltage by varying resistancewhen the device is switched between the first and second states.
 14. Adevice as claimed in claim 13 wherein the device includes receivingmeans in series with the series regulator, the receiving means beingconfigured for receiving a signal.
 15. A device as claimed in claim 1wherein the device includes a load in series with the series regulator.16. A device as claimed in claim 1 wherein the device includes a shuntregulator to control the operating voltage when the device is switchedbetween the first and second states.
 17. A device as claimed in claim 1wherein the voltage multiplier transforms the load impedance by a factorof
 8. 18. A device as claimed in claim 1 wherein the rectifiertransforms the load impedance by a factor of
 2. 19. A device as claimedin claim 1 wherein the device is configured for controlling current andwherein there is a relatively smaller amount of current, relative to thefirst state, when the device is in the second state.
 20. A device asclaimed in claim 19 wherein in the first state the current is hundredsof microamperes and in the second state the current is tens ofmicroamperes.
 21. A device as claimed in claim 19 or 20 wherein in thesecond state the relatively smaller amount of current is less thanapproximately 50 μA.
 22. A device as claimed in claim 19 or 20 whereinin the second state the relatively smaller amount of current is lessthan approximately 30 μA.
 23. A device as claimed in claim 19 wherein inthe second state the relatively smaller amount of current is less thanapproximately 15 μA.
 24. A device as claimed in claim 19 or 20 whereinin the second state the relatively smaller amount of current is betweenapproximately 1 μA and approximately 4.99 μA.
 25. A device as claimed inclaim 19 wherein in the second state the relatively smaller amount ofthe first current is less than 50% of the relatively larger amount ofthe first current.
 26. A device as claimed in claim 1 wherein theswitched impedance is configured to select the second state morefrequently than the first state.
 27. A device as claimed in claim 1wherein the switch is used to select the first or second statesaccording to an algorithm.
 28. A device as claimed in claim 1 wherein inthe second state the device acts as a full wave bridge rectifier.
 29. Adevice as claimed in claim 1 wherein the voltage multiplier provides asincreased output voltage.
 30. A device as claimed in claim 1 wherein inthe first state the device acts as a voltage doubler.
 31. A device asclaimed in claim 1 wherein the device is utilized in a radio frequencyidentification device.
 32. A device as claimed in claim 1 wherein theradio identification frequency device is passive.
 33. A device asclaimed in claim 1 wherein in the second state current is used tomaintain RAM data stored in CMOS memory, and operate logic functions.34. A device as claimed in claim 1 including an onboard energy storagedevice.
 35. A method of selectively controlling current, the methodcomprising: providing a device operable in one of a first state or asecond state, coupling an impedance to the device to enable switching ofthe device between a first state and a second state wherein, in thefirst state, the device acts as a voltage multiplier and, in the secondstate, the device acts as a rectifier.
 36. A method as claimed in claim35 wherein the method includes using a receiving means to receive asignal, and selectively controlling the amount of current in thereceiving means by switching between the first state and the secondstate.
 37. A method as claimed in claim 36 wherein the current drawnthrough the receiving means in the first state is greater than thecurrent drawn through the receiving means in the second state.
 38. Amethod as claimed in claim 36 or 37 wherein the receiving means isconnected to one or more voltage input terminals of the device.
 39. Amethod as claimed in claim 36 wherein the receiving means comprises acoil in an antenna circuit.
 40. A method as claimed in claim 36 whereinthe receiving means has a impedance of substantially 200 ohms.
 41. Amethod as claimed in claim 35 wherein a load is connected between one ormore output voltage terminals of the rectifier.
 42. A method as claimedin claim 41 wherein the load comprises an integrated circuit.
 43. Amethod as claimed in claim 35 wherein the first state is an operationalstate and the second state is a standby state.
 44. A method as claimedin claim 35 wherein the switched impedance comprises a switch in serieswith a capacitor.
 45. A method as claimed in claim 44 wherein the switchof the switched impedance comprises a MOSFET.
 46. A method as claimed inclaim 35 wherein the switched impedance is connected between a firstinput terminal and a first output terminal of the rectifier.
 47. Amethod as claimed in claim 35 wherein the device includes a seriesregulator for controlling an operating voltage by varying resistancewhen the device is switched between the first and second states.
 48. Amethod as claimed in claim 47 wherein the device includes receivingmeans in series with the series regulator, the receiving means beingconfigured for receiving a signal.
 49. A method as claimed in claim 35wherein the device includes a load in series with the series regulator.50. A method as claimed in claim 35 wherein the device includes a shuntregulator to control the operating voltage when the device is switchedbetween the first and second states
 51. A method as claimed in claim 35wherein the voltage multiplier transforms the load impedance by a factorof 8
 52. A method as claimed in claim 35 wherein the rectifiertransforms the load impedance by a factor of
 2. 53. A method as claimedin claim 35 wherein the device is configured for controlling current andwherein there is a relatively smaller amount of current, relative to thefirst state, when the device is in the second state.
 54. A method asclaimed in claim 53 wherein in the first state the current is hundredsof microamperes and in the second state the current is tens ofmicroamperes.
 55. A method as claimed in claim 53 or 54 wherein in thesecond state the relatively smaller amount of current is less thanapproximately 50 μA.
 56. A method as claimed in claim 53 or 54 whereinin the second state the relatively smaller amount of current is lessthan approximately 30 μA.
 57. A method as claimed in claim 53 or 54wherein in the second state the relatively smaller amount of current isless than approximately 15 μA.
 58. A method as claimed in claim 53 or 54wherein in the second state the relatively smaller amount of current isbetween approximately 1 μA and approximately 4.99 μA.
 59. A method asclaimed in claim 53 wherein in the second state the relatively smalleramount of the first current is less than 50% of the relatively largeramount of the first current.
 60. A method as claimed in claim 35 whereinthe switched impedance is configured to select the second state morefrequently than the first state.
 61. A method as claimed in claim 35wherein the switch is used to select the first or second statesaccording to an algorithm.
 62. A method as claimed in claim 35 whereinin the second state the device acts as a full wave bridge rectifier. 63.A method as claimed in claim 35 wherein the voltage multiplier providesas increased output voltage.
 64. A method as claimed in claim 35 whereinthe voltage multiplier is a voltage doubler.
 65. A method as claimed inclaim 35 wherein the device is utilized in a radio frequencyidentification device.
 66. A method as claimed in claim 35 wherein theradio identification frequency device is passive.
 67. A method asclaimed in claim 35 wherein in the second state current is used tomaintain RAM data stored in CMOS memory, and operate logic functions.68. A method as claimed in claim 35 including an onboard energy storagedevice.
 69. A method as claimed in claim 35 wherein the coupling of theimpedance is enabled by a switch.